1. Field of the Invention
The present invention relates to starting and driving permanent magnet motors.
2. Description of the Prior Art
Permanent magnet motors are utilized in inertial gyroscopic sensing instruments where optimum efficiency and rate stability are required.
In prior applications, gyroscope wheels were supported by a ball bearing suspension where bearing torque differential between start and run mode is relatively minor. Current applications requiring high performance and low noise operation utilize gyroscopes where the rotating inertia is suspended on a hydrodynamic bearing. In a hydrodynamic bearing, the rotating member is constructed to generate the pressurized gas utilized by the bearing. These type of bearings are characterized by a relatively high dry friction start up torque and virtually zero friction running torque. Thus, the torque dynamics required to provide reliable start/run operation become very critical.
In conventional permanent magnet wheel drive designs, the permanent magnet drive motor includes a two-phase stator, a permanent magnet rotor and a suitable electronic drive stage to start, run-up and maintain a synchronous speed operation. Starting is implemented using a two-phase drive mode during which drive current is fed to both stator windings. At a suitable rotational rate, typically 5% to 10% of synchronous speed, the drive mode is switched to single phase mode wherein drive current is fed to only one stator winding. The other stator winding functions as a sensing winding and provides a voltage which is indicative of rotor position and speed. The driven phase winding is driven by a signal which is a function of both the speed signal from the sensing winding as well as the input speed command reference. The speed signal is commutated by the position signal from the sensing winding. Upon achieving synchronous speed, steady state acceleration torque becomes zero and the amplitude of the drive current is reduced by an appropriate feedback signal to the minimum required for the motor to overcome friction and windage torques. Prior art U.S. Pat. Nos. 4,275,343 and 4,492,902 show exemplary permanent magnet motor starting circuits.
In order to commence motor rotation from rest and drive it up to a predetermined rotational velocity, typically a small fraction of synchronous speed, a starting circuit is provided that slowly increases the excitation frequency until an appropriate speed is reached at which back EMF feedback control can effectively operate. From this point on the drive electronics switches into a single phase mode drive operation. The back EMF is now used to provide the following: (a) speed information to be compared to reference speed command signal; and (b) timing information for single phase drive commutation.
In conventional permanent magnet motor applications, starting is implemented in an open loop manner, without feedback. If feedback is not provided, wheel starting characteristics are indeterminate and a no start condition can occur under adverse environmental conditions. The present invention avoids such problems by generating a rotating magnetic field which works in conjunction witn sense and decision circuits to determine whether a run mode is to be maintained or a restart mode is to be initiated.